This invention relates to an enclosure from which magnetic and electrical fields are excluded, and, more particularly, to such an enclosure constructed of superconducting materials.
The human body produces various kinds of energy that may be used to monitor the status and health of the body. Perhaps the best known of these types of energy is heat. Most healthy persons have a body temperature of about 98.6.degree. F. A measured body temperature that is significantly higher usually indicates the presence of an infection or other deviation from normal good health. A simple medical instrument, the clinical thermometer, has long been available to measure body temperature.
Over 100 years ago, medical researchers learned that the body also produces electrical signals. Doctors today can recognize certain patterns of electrical signals that are indicative of good health, and other patterns that indicate disease of abnormality. The best known types of electrical signals are those from the heart and from the brain, and instruments have been developed that measure such signals. The electrocardiograph measures electrical signals associated with the operation of the heart, and the electroencephalograph measures the electrical signals associated with the brain. Such instruments have now become relatively common, and most hospitals have facilities wherein the electrical signals from the bodies of patients can be measured to determine certain types of possible disease or abnormality.
More recently, medical researchers have discovered that the body produces magnetic fields of a type completely different than the other types of energy emitted from the body. The research on correlating magnetic fields with various states of health, disease and abnormality is underway, but sufficient information is available to demonstrate that certain emitted magnetic fields are associated with conditions such as epilepsy and Alzheimer's disease. Ongoing medical studies are investigating the nature of the normal and abnormal magnetic fields of the brain, and seeking to correlate those fields with the precise location in the brain from which they emanate. If it were known that a particular abnormality, such as Alzheimer's disease, were associated with an abnormal magnetic field produced at a particular location in the brain, it might be possible to detect the abnormality at an early stage, while it was treatable, and then apply other medical knowledge to treat that precise portion of the brain. Magnetic studies of the brain therefore offer the potential for understanding and treating some of the most crippling diseases and conditions known.
The biomagnetometer is an instrument that has been developed for measuring magnetic fields produced by the body, particularly the brain. The biomagnetometer is a larger, more complex instrument than the medical instruments mentioned earlier, primarily because the magnetic fields produced by the body are very small and difficult to measure. Typically, the strength of the magnetic field produced by the brain is about 0.00000001 Gauss. By comparison, the strength of the earth's magnetic field is about 0.5 Gauss, or over a million times larger than the strength of the magnetic field of the brain. Most electrical equipment also produces magnetic fields, in many cases much larger than that of the earth. It is apparent that, unless special precautions are taken, it is difficult or impossible to make magnetic measurements of the human body because the external influences such as the earth's magnetism and nearby apparatus can completely mask the magnetic fields from the body.
The biomagnetometer includes a very sensitive detector of magnetic signals. The currently most widely used detector is a Superconducting Quantum Interference Device or SQUID, which is sufficiently sensitive to detect magnetic signals produced by the brain. (See, for example, U.S. Pat. Nos. 4,386,361 and 4,403,189, whose disclosures are incorporated by reference, for descriptions of two types of SQUIDs.) This detector and its associated equipment require special operating conditions such as a cryogenic dewar, and cannot be placed into the body or attached directly to the surface of the body.
The present biomagnetometer usually includes a chair or table upon which the patient is positioned, and a structure which supports the SQUID in proximity with the head of the patient, as about 8 inches away. Special electronics are used to filter out external effects such as the earth's magnetic field and the magnetic fields of nearby electrical instruments. (For a description of such a device, see U.S. Pat. Nos. 3,980,076 and 4,079,730, whose disclosures are herein incorporated by reference.) The electronics filters out a portion of the external magnetic noise, but in some regimes is not entirely successful. The electronics is also costly and amount to a major portion of the cost of the system.
There is another possibility for reducing the adverse effect of the external magnetic field, which can be used in place of, or in addition to, the electrical signal processing. In this approach, the patient and detector are placed into a magnetically quiet enclosure that shields the patient and the detector from the external magnetic fields. The magnitude of the magnetic field within the enclosure is reduced from about 0.5 Gauss or more, to less than about 0.001 Gauss. With this reduction in the ambient magnetic field, the magnetic events of interest can be measured more readily, and the signal processing required to achieve usable information is greatly reduced.
Magnetically shielded enclosures have been known, as for example the design description in U.S. Pat. No. 3,557,777, whose disclosure is herein incorporated by reference. In this approach, concentric layers of a high permeability metal and a metallic conductor are supported on a frame. While operable, such enclosures are heavy and depend upon the use of expensive, high permeability metals that must be procured specially. The preparation of each such enclosure is therefore essentially a custom operation, requiring long lead times, and the enclosures are costly.
Accordingly, there exists a need for an improved magnetically shielded enclosure which has a low level of electromagnetic and magnetic noise in its interior, and which is less expensive to construct than existing enclosures. The present invention fulfills this need, and further provides related advantages.